Optical amplifiers are important in networks for distributing optical signals. Optical fiber amplifiers, made from glass that is doped with rare earth elements such as erbium, are a well-known example. For example, U.S. Pat. No. 5,119,460, issued to Bruce, et al. on Jun. 2, 1992, describes optical amplifiers with relatively high rare earth content. The optical amplifiers disclosed therein are in the form of planar waveguides deposited on silicon substrates. Planar waveguides are useful for applications such as premises distribution of optical signals, where components need to be relatively small and device integration is desirable. The optical amplifiers described in Bruce et al. contain alkali metals, alkaline earth metals, or a combination thereof, to prevent clustering of erbium atoms.
Although alkali and alkaline earth metals are effective for this purpose, glasses containing significant amounts of these materials are not entirely complementary with the silicon substrates on which they are deposited. For example, glasses that contain significant amounts of alkali and alkaline earth metals have a higher coefficient of thermal expansion than the silicon substrate, which can cause stress in the glass films when the glasses are subjected to changes in temperature typically associated with annealing and waveguide fabrication. This stress increases with film thickness and can cause films of alkali-containing and alkaline earth-containing materials such as soda-lime glasses that are greater than 2 .mu.m thick to crack. Also, waveguides with alkali and alkaline earth metals cannot tolerate temperatures greater than 900.degree. C. without deforming. This limits the choice of material for the upper cladding layers to materials that can be formed on the glass at temperatures less than 900.degree. C.
Also, after the alkali or alkaline earth metal-containing glass is formed on the substrate, there can be out-diffusion of the mobile alkali metals. This can adversely affect the adhesion to the glass of the photodefinable material used to define the pattern in the waveguide structure. This out-diffusion can also adversely affect the homogeneity of the glass film and increase the defect density of the film. Also, since alkali and alkaline earth metals are detrimental to some other processes for making silicon-based devices (e.g. integrated circuits) equipment that is used to form these glasses on silicon substrates cannot be used for processing some other silicon-based devices. Consequently, glasses for optical amplifiers and optical fibers which do not contain alkali metals and alkaline earth metals are desired.